In acoustically noisy environments, listeners often have difficulty hearing a desired audio signal or “signal-of-interest”. For example, a cellular phone user in an automobile may have difficulty understanding the received speech signal through their headset because the noise of the automobile masks the signal-of-interest (i.e., the speech signal received by the cell phone). Many attempts have been made in the past to solve this problem. Some of them are described briefly as follows:
(a) Passive noise attenuating headsets: For the specific application in headset applications, passive noise attenuation is provided by a large and bulky ear cup that physically isolates the environmental (acoustic) noise from the users ear.
(b) Amplification: The incoming electrical signal-of-interest is amplified to overcome the background noise level. If not properly controlled, this can result in dangerously loud output levels. Also, unless the amplification well-controlled it may not provide the desired benefit.
(c) Filtering: The signal is statically filtered to make it more intelligible
(d) Simple Automatic Gain Control (AGC): The signal-of-interest is passed through an automatic gain control (AGC) system in which gain is adjusted based on a level measurement of the noise inside or outside the ear cup. The gain of the AGC is typically controlled by a simple measurement of the overall noise level.
(e) Active noise cancellation (ANC): Anti-noise (generated using either an open- or closed-loop servo system) is generated and added acoustically to the noise signal. For headset applications, sec Bose, Amar, et. al. Headphoning, U.S. Pat. No. 4,455,675. Jun. 19, 1984. and Moy. Chu. Active Noise Reduction in Headphone Systems, Headwize Technical Paper Library, 1999.
(f) Sometimes, these methods are combined: a common scheme for a headset application is to combine a passive noise-attenuating headset with an ANC system (see Bose, Amar, et. al. Headphoning, U.S. Pat. No. 4,455,675. Jun. 19, 1984).
Although these methods are highly effective and reduce the noise for a wide range of applications, they are not always suitable. For example, ANC-requires an accurate noise reference, which may not be available and works only at lower frequencies. Passive noise reduction works well only if sufficient room is available for the sound insulation. Filtering distorts the signal frequency content. AGC systems do not consider the human auditory system and yield sub-optimal results Also, even when these solutions can be applied, applications exist where the power drain of these solutions is prohibitive and a miniature, low power technique is required.
Accordingly, there is a need to solve the problems noted above and also a need for an innovative approach to enhance and/or replace the current technologies.